Early 3rd Gen pistols, ending in xxx4 or xxx5 were simply blued carbon steel. The bluing wore easily in a holster.

Early attempts to blacken the stainless xxx6 models included Black-T and another baked on finish. Neither was acceptable for hard use (cosmetically).

Melonite appears to be the best answer, and numerous experimental 3rd Gen models have turned up during in recent years. The first run was the non-TSW 4566's from about 10 years ago. I believe this led to Melonite being chosen for the M&P series.

I have had it applied to several slides, frames and barrels with excellent results. It really likes stainless. Below are some pics. I don't know of anyone applying it to small parts. Possibly because of the size, but mostly because of the metallurgy.

Stainless Beretta slide with Melonite:


Carbon steel Beretta slide with Melonite:


Stainless Barrel:


Factory 5946


Aftermarket 4586TSW


Aftermarket 4553TSW slide (black anodized frame)